The invention relates to means for positioning of electronic components on printed wiring boards. More specifically, the invention relates to an apparatus for precision positioning of individual electronic components on printed wiring boards.
FIG. 8 is an illustration of the optical path and functional principle of a known fine placement apparatus using a video camera for observing placement of a component on a component carrier. Component 100 which is to be placed on a component carrier 101 is held in a component fixture 102 at the end of a placement rod 103. Placement rod 103 can be moved in a direction normal to component carrier 101. Rod 103 moved through opening 104 in mirror 105. The movement of rod 103 allows to load a component 100 on to fixture 102 of rod 103. Rod 103 is then lowered towards component carrier 101. The image of the top side of component 100 and the placement area surrounding on component carrier 101 around component 100 is mirrored by mirror 105 onto a dual mirror 106, where the image is reflected into the optic of vertical camera 107. This arrangement can only match the leads of component 100 with the foot print pads on component carrier 101 as far as these pads are not covered by the terminals of component 100. In a version for placing large components mirror 106 is implemented as dual mirror with the common edge of the two partial mirrors 108a and 108b being turned 45 degree relative to the projected image of component 100. In this arrangement component 100 is out of focus until it is just above the placement location. The placement location on component carrier 101 is covered by component 100 until the same is close to the placement location. At this time component carrier 101 has to be positioned so that the terminals of components 100 match the foot print pattern of the placement location on component carrier 101. The placement of components with terminals underneath the package provides additional difficulties, because none of these terminals are visible during the placement process.
The German Patent DD 242 320 discloses an apparatus for positioning of electronic components having a large number of terminals, especially surface mount components on printed wiring boards. In this publication the printed wiring board is mounted on an x-y table and the component is held by vacuum on the holding fixture of a pivoted placement arm. The under side of the component and the top surface of the printed circuit board are illuminated by a light source. A semi reflective optical mirror provides a combined real image of the component and the position at which the placement arm could position the component. The combined images can be observed through a microscope. By proper positioning of the printed circuit board using the controls of the x-y table the image of the desired positioning location for the component on the printed circuit board and the under side of the component held in the holding fixture of the positioning arm are matched. Without further change in the position of the printed circuit board the positioning arm is lowered onto the printed circuit board placing the component in the desired position. Releasing the vacuum allows to lift the positioning arm away from the component.
While this apparatus functions satisfactorily the arrangement of optical components and positioning arm is complex. It was discovered that positioning was not always as precise as expected despite proper adjustment of the images of the desired position on the printed circuit board and the electronic component.
With increased size of the components it became apparent, that a new method had to be found for matching the images of the desired position on the printed circuit board and the electronic component. It was not anymore feasible to use the complete image of the component. Furthermore, observing the images through a microscope was rather strenuous to the observing eye.
By projecting the images onto a video camera the images are observable on a display monitor. The problem of this method is the low resolution of the camera or the display monitor; the large number of the narrowly spaced leads of the component interferes with the display lines making precise matching of the images of the component difficult.